The frame of a dynamoelectric machine contains at its inner periphery an array of keybars, i.e., long metallic rods formed with a bolt portion that compresses the core and mounts the laminations in its dovetail slot. The core is a long hollow cylinder, made of hundreds of thousands of laminations that are punched from an insulated silicon steel strip to a segment shape. At the periphery, each lamination has dovetail slots, one of which engages the dovetail portion in the keybar. The laminations are mounted over the keybars in a circular fashion to form a first annular planar array, each separated by a small air gap called a segment gap. A second annular array is stacked on top of the first and offset from it in a brick wall fashion. Groups of such annular arrays of laminations that are formed either outside or inside a core-pit are termed a “core packet”. Multiple core packets separated by spacer bars that create cooling ventilation ducts are stacked axially to form a core stack. A typical core stack is 3 to 10 m (10 to 30 ft) long, comprising 2 to 8 cm (1 to 3 inches) thick core packets, each separated by 0.3 to 1.5 cm (0.125 to 0.5 inches) thick vent ducts, see U.S. Pat. No. 7,567,788. The stator core is formed by clamping the stack between two flanges using the keybars. The core provides a low reluctance path for a rotating magnetic field, which is generated by coils within a rotor that rotates inside the core bore. Under certain extreme operating and manufacturing conditions, the magnetic fields can cause localized heating and melting of the core in a phenomenon known as a core failure. The results of a few known core failures are summarized below.
In 1968, in a power plant operated by Britain's CEGB, a 500 MW generator core was damaged by melting per Fairney (1989). In a simulation of this 500 MW generator, Tavner and Anderson (2005) predicted that keybars in a shorted core could carry thousands of amperes of currents comparable to those carried by the stator bars. In 1998, a 300 MW generator in San Antanio, Tex., overheated because of numerous interlaminar shorts and was removed from service per Spisak (2004). In 2000, a core failure occurred in a 415 MW generator in Castle Dale, Utah, in which about 200 pounds of molten iron flowed out of the core-end, the cause for which was traced to an interlaminar short which grew into a major melt zone per Edmonds et al (2007). These in-depth studies pointed to excessive eddy currents created by shorts as the root cause of core failures.
That the main flux can initiate a core failure is well known [see U.S. Pat. Nos. 4,494,030; 5,252,915 and Edmonds 2007]. FIG. 1-A shows a keybar 10 and laminations 20, 21 with a defect at d at the insulation interface 16 of the stack. The laminations 20, 21 are in electrical contact with the keybar 10 at the points b, f. As the main flux φθ carried by laminations rotates, it induces eddy voltage Vθ=dφθ/dt. If Vθ is greater than the dielectric breakdown voltage Vb of the insulation, then the insulation shorts at d, driving eddy current in a loop bcdef through the keybar 10 and laminations 20, 21. When laminations are shorted, the heat increases in proportion to square of number of shorted laminations, termed multiplier effect. Further, there is no mechanism to remove this eddy heat to the coolant gas. So when large eddy currents flow through a small shorted spot, the local temperature shoots up and burns the insulative coating in adjacent laminations, causing additional axial shorts. As more laminations short axially, the multiplier effect sharply increases the eddy current, increasing the heat, which in turn expands the melt zone axially. The insulation shorts thus amplify eddy currents and expand the melt zone uncontrollably, resulting in a core failure. From FIG. 1-A it can be seen that the keybar/lamination contacts b, f play a pivotal role and make it possible for the eddy currents to close the loop and cause core failure. If these keybar/lamination contacts b, f are broken—by isolating the keybars from laminations—the multiplier effect is suppressed and far less eddy current, confined to individual laminations, will flow, reducing the core failure risk.
That leaking fluxes can also initiate core failure, especially at the core-end, is shown in FIG. 1-B. This figure shows a pair of laminations 21, 22 in a circular array at the core-end. They are electrically connected to a pair of keybars 10, 26 via electrical contacts at y, r. Keybars 10, 26 are also welded to a support ring 28 making electrical contacts at n, p. The segment (or radial edge) 23 of lamination 21 and segment 24 of adjacent lamination 22 are separated by a segment gap 25. Laminations 21, 22 can have two kinds of defects: a segment defect such as t along segments, and an insulation defect such as h along stack. Rotating radial leakage component of flux φr induces a small voltage differential (called keybar voltage Vr) between adjacent keybars 26, 10. The rotating fringe flux φz also induces a fringing voltage Vz. Both Vr, Vθ combine vectorially to form a larger resultant Veff. The paths in which resulting currents flow depend on which parts are in electrical contact. Here keybars are assumed to be in contact with laminations. If Veff is greater than the dielectric breakdown voltage Vb, laminations will have insulation short at h and segment short at t. The insulation short at h will cause eddy currents to flow through the six-legged loop ynprghky containing the insulation short h The segment short at t will cause additional eddy currents to flow through the six legged loop ynprstuy containing the segment short t. Both currents flow through the keybars so keybars will see very large eddy currents. Such large eddy currents flowing through keybars, laminations and flanges (if they are not isolated) cause arcing, pitting and core-end heating per U.S. Pat. Nos. 6,720,699, 6,713,930, and 6,462,457 precipitating a core failure or force one to operate at a lower power level. From FIG. 1-B, it is clear that the keybar/lamination contacts y, r play a pivotal role in enabling the eddy currents to close the loops and cause core failure. If these keybar/lamination contacts y, r are broken—by isolating the keybars from laminations—a far less eddy current confined to individual laminations will flow, reducing the core failure risk.
Preceding analysis reveals that two conditions are necessary for core failure: first, effective eddy voltage Veff must be higher than the dielectric breakdown voltage Vb and second, the keybars should make electrical contact with the laminations. Eliminating one of these conditions could beneficially protect the machine against core failures. Since one cannot eliminate all electrical defects (that control Vb)—as hundreds of thousands of laminations are involved—we focus on eliminating the electrical contact of keybars with the laminations (as only a dozen or so keybars are involved). A natural conclusion is that, if the keybar is designed deliberately to be electrically isolated from the laminations, then the risk of core failure is reduced. Then, since limited eddy current flows within individual laminations, the efficiency is increased.
To protect the machine from core failure, direct methods that redirect or cut-off eddy currents (such as conductive cage, insulated keybar) and other indirect methods (such as recoating, overflux monitoring, core end stepping, flux shield, flux shunt, short rotor, higher-grade iron etc) are currently employed. In the recoat method, the core-end laminations are recoated once or twice (after punching and deburring), to nearly double the insulation thickness, hopefully increasing its dielectric breakdown voltage Vb above the eddy driving voltages, hence reducing the core failure risk. Even though laborious and expensive, recoating is the most widely used and proven method. However, the recoat layer itself may embed conductive particles such as iron debris from deburring operation or slivers from punching operation, which reduces the dielectric breakdown voltage, increasing the core failure risk.
Focussing on the direct methods, in the conductive cage method, one inserts a copper strap into each dovetail slot between keybars and laminations (see U.S. Pat. Nos. 6,462,457 and 6,720,699) and coupler wires to connect all straps electrically (see U.S. Pat. No. 6,429,567) to form a conducting cage that surrounds the core to which eddy currents due to leakage flux are hopefully redirected. The straps short all laminations to keybars. However, such shorting of keybars to all laminations unwittingly closes eddy loops and generates more eddy currents, increasing the core failure risk as already described. Further, there is no easy path to transfer the eddy heat from the trapped straps into a cooling medium, so the straps become very hot rapidly. In view of these problems, the conductive cage method is not widely used by the industry.
In the insulated keybar method an insulative media is inserted in the slot between the keybar and laminations (in contrast, a conductive media is inserted in the conductive cage method). This insulative media breaks all eddy current paths between keybar and laminations, reducing the core failure risk and increasing the efficiency. In prior art of the insulated keybar method, three approaches are used: a one-piece insulator, a two-piece insulator and heat shrinkable tubing. U.S. Pat. No. 4,494,030 teaches a one-piece insulator, shaped like a cylindrical tube, covering loosely a round keybar and supported by two thick non-magnetic end plates. However, the non-magnetic plates are exposed to fringe fluxes, so eddy losses will increase, reducing the efficiency. Alternatively, U.S. Pat. No. 7,202,587 describes a two-piece insulator, comprising wedges and a thermoplastic insulative strip in the dovetail slot; wedges tighten the insulative strips. However, the soft insulator strips cannot withstand the severe shear stresses caused by torque forces and can tear apart. Further, the insulating strips can wear out due to abrasive vibrations. A parallel technology taught in U.S. Pat. No. 6,949,858 uses thick heat shrinkable tubing to insulate a through-bolt. However, the keybar's dovetail and bolt portions join at sharp concave corners, which prevents use of shrink tubing to insulate keybars. Because of these limitations of prior-art, it is beneficial to develop new methods of insulation that can withstand the severe shear stresses and abrasive wear of vibratory loads over several years of operating life.
As seen in FIG. 1-C, in prior art, the keybar 10 is a solid steel rod that has a dovetail portion 12 (that engages the slot 39 in a core packet 49) below the corners 71, 72 and a rectangular bolt portion 11 (that engages the support rings 28 and flanges) above the corners 71, 72. FIG. 1-D indicates that the slot 39 in the core packet 49 has slant faces 43, 44 and a flat face 45. The gaps 46, 47, 48 relate the slot faces 43, 44, 45 with respective dovetail faces 40, 41, 42. FIG. 1-C shows a condition in which the slots are bigger than dovetail, so none of the faces of slots 39 contact with respective faces of the dovetail 12 when the slots are centered about the dovetail. There are clear gaps 46, 47, 48 between respective faces. The dovetail/slot gap designer encounters conflicting requirements for designing these gaps 46, 47, 48. For easy assembly, sufficient gap 46, 47, 48 must be provided between respective faces of the dovetail and the slot. However, during operation, for transmitting torque forces, there must be no gap between at least one pair of respective faces (i.e., they must contact each other). A few solutions were proposed in the prior art to address this dilemma. In an earlier U.S. Pat. No. 6,448,686, Dawson used a first prepackaged core packet made of laminations with non-contacting slots, and a second manually stacked core section made of laminations with contacting slots. The non-contacting slots enable one to insert and center the prepackaged core packet over the dovetails. All these non-contacting laminations are held to the frame by the stacking pressure. However, it was found that the stacking pressure reduces as the machine ages, leading to loose laminations that rattle. Hence, in later U.S. Pat. Nos. 6,597,081 and 6,775,900 Dawson discarded the concept of concentrating non-contacting slots in prepackaged core packets; instead, laminations with non-contacting slots are intermixed with those which have contacting slots, with intermixing carefully controlled to distribute the contact points evenly. Obviously, such selective positioning of contact points evenly among hundreds of thousands of laminations is very labor intensive and hence expensive. Further, this approach still did not solve the loose lamination problem, as non-contacting laminations are still present in the stack and can become loose and rattle. To overcome this rattling laminations problem, the patent application 2011/0109187 by Tanavde teaches compression bands (comprising a strap, cable and tightening means) or belts around the keybar cage, whose tightening forces the dovetails to contact and press against lamination slots, thereby removing the looseness and preventing laminations from rattling. However, this belt-tightening forces the keybar to contact laminations, which enables eddy currents in shorted laminations to close the paths, thereby increasing the eddy currents and hence has the unfortunate side-effect of increasing the core failure risk and reducing the efficiency. All these problems of rattling laminations, core failure risk, reduced efficiency and the added cost of manufacturing and installing the compression bands could be eliminated by attaching the laminations to insulated keybars (i.e., respective slanted faces press against each other tightly) in the first place as proposed in the present invention.